This application claims the priority of Japanese Patent Application No. 3-357763 filed Dec. 26, 1991, which is incorporated herein by reference.
The present invention pertains to an apparatus for assembling a nuclear fuel assembly.
FIG. 6 depicts a known nuclear fuel assembly which is mounted on a nuclear reactor such as a pressurized water reactor. The assembly, generally designated at 1, includes a pair of top and bottom nozzles 2 and 3 arranged in a facing relation to each other and in a vertically spaced relation to each other. A plurality of guide pipes 4 such as control-rod guide pipes or instrument pipes are disposed so as to extend between the top and bottom nozzles 2 and 3, and fixedly secured thereto. A plurality of grids 4 are secured to the intermediate portions of the guide pipes 5 so as to be vertically spaced from one another, and a plurality of fuel rods 6 are inserted through and supported by the grids 4 so as to extend parallel to the guide pipes 5.
As shown in FIGS. 7 and 8, each grid 5 is formed as follows. A plurality of straps 11, each in the form of a thin strip sheet, are assembled perpendicularly to each other into a grid 5 to define a number of grid cells 13. Springs 15 and dimples 16 for supporting a respective fuel rod 6 are formed on the walls of each grid cell in opposed relation to each other. More specifically, as shown in FIG. 8, a single spring 15 is formed on one side of each wall of the grid cell 13, while a pair of dimples 16 are formed on the other side of the same wall defining adjacent grid cell so as to sandwich the aforesaid spring 15, and a spring 15 and a pair of dimples 16 are opposed to each other and protrude into the same grid cell 13. Each fuel rod 6, which is inserted into the grid cell 13, is supported by being urged to the dimples 16 by the spring 15 opposing thereto. Furthermore, rectangular cut-outs 14 or openings are formed at the intersections of the straps 11. Those wall portions which sandwich the spring 15 therebetween and are spaced apart from each other in the longitudinal direction of the strap serve as ribs 23 with which hook portions 22 of a key member 21 are held in engagement.
A conventional method for inserting the fuel rods 6 in the grid 5 thus constructed will now be described.
First, a deflecting jig 24 as shown in FIG. 9 is inserted through the grid cell 13. The deflecting jig 24 comprises a cylindrical jig body 25 divided at its forward portion into four pieces, and a tapered pin 26 inserted in the jig body 25 for sliding movement therealong. When the tapered pin 26 is withdrawn or retracted in its axial direction while holding the deflecting jig 24 in the grid cell 13, the jig body 25 is enlarged to deflect the springs 15 as shown in FIG. 10.
Subsequently, an elongated key member 21 of a generally rectangular cross-section is inserted into the cut-outs 14 from the lateral side of the grid 12, to bring the hooks 22 of the key member 21 into engagement with the ribs 23 to keep the springs deflected. Then, the deflecting jig 24 is removed from the grid.
Thereafter, the fuel rod 6 is inserted in the grid cell 13, and the key member 21 is removed from the grids to release constriction of the spring. Thus, the fuel rod 6 is secured by being resiliently urged by the springs 15 toward the dimple 16 opposing thereto.
However, inasmuch as the aforesaid tasks of deflecting the springs 15 and inserting the key member 21 are carried out manually, they are labor-intensive and time-consuming, resulting in low operational efficiency.
Furthermore, the task of deflecting the springs 15 using the aforesaid deflecting jig 24 must be carried out carefully so as not to cause any twisting or shifting to the springs 15, without exerting any force thereon in a direction other than the acting direction of the spring 15. Therefore, the deflecting task requires a high level of skill, so that it has been desired to improve this task and achieve standardization of the operating procedure.